1. Field of the Invention
The present invention generally relates to actuators used for performing mechanical work and, more particularly, to fluidic artificial muscles, artificial muscle actuators, or McKibben artificial muscles.
2. Description of Prior Art
Fluidic muscle actuators (also known as artificial muscle actuators, or McKibben artificial muscles, among other names) are simple mechanical actuators that harness pressurized fluid (air, water, oil, etc.) to generate significant forces and deflections. They have attracted interest in the fields of robotics, industrial automation, and recently aerospace engineering (U.S. patent application Ser. No. 11/502,360) because of their simple design, light weight, compliance, and excellent performance in terms of forces and deflections generated.
The operating principle of fluidic muscles is as follows. The inner elastic bladder is pressurized with the operating fluid (air, water, oil etc.), causing an inflation and expansion of the bladder. The braided sleeve around the bladder is thereby forced to expand; however, the fixed length of the stiff sleeve fibers generates either a tensile or a compressive force along the main axis of the actuator, in addition to relative motion between the two end fittings, either contraction or extension, but not both. The direction of force and motion are dependent on the initial angle between the filaments of the braided sleeve. Actuators with the filaments of the sleeve aligned primarily with the length axis of the actuator will be contractile. If the filaments of the sleeve are aligned primarily with the radial axis of the actuator, then the motion is extensile. There is a neutral braid angle in between these two extremes that is the cross-over point between these two regimes. Typically this angle has been found to be 54°44′ as measured from the length axis of the actuator to the braid filaments. For a tension/contraction actuator, the bladder expansion is radial and for a compression/extension actuator, the bladder expansion is primarily axial. The direction of force and motion is inherent to the construction of a given actuator and can not be changed once the actuator is made. This force and motion is transferred to an external system via the end fittings.
Fluidic muscle actuators of this type have been known in prior patent publications. A related device was disclosed in April 1957 in U.S. Pat. No. 2,789,580. Many different designs have been disclosed over the years (U.S. Pat. Nos. 2,844,126, 4,733,603, 4,751,869, and 5,021,064). Some more recent designs, such as those disclosed in (U.S. Pat. Nos. 4,615,260 and 6,349,746 B1), are commercially available.
The vast majority of devices known in the prior art have the braided sleeve configured to generate tensile forces and contractile motions. This is partly because the tension force levels that can be generated in the contractile configuration are much larger than the compression forces generated with the alternate configuration.
However, for many current and future applications of these actuators, contractile motion is not preferred. For example, an industrial stamping process where an extensile actuator is needed to push two plates together, thereby creating a compressive force between them. If extensile motion such as this is desired, then current Fluidic Muscle Actuator designs become less attractive because they will either lose much of their force generation potential if an extensile braid angle configuration is used, or they will lose their simplicity advantage if some additional complex mechanism is needed to convert the motion of a contractile braid angle actuator into extensile output motion.
Therefore, what is desired is a new design of Fluidic Muscle Actuator which combines the high force capability of a contractile braided sleeve configuration with compressive force generation and extensile motion output. The current invention accomplishes this goal with a supplementary motion conversion package that can be added or removed from the actuator with little effort, and that changes the direction of force and motion with only a small increase in friction, weight, and cost.